The prior art and related art arrangements occur in the context of a larger wireless telecommunications system, as exemplified by FIG. 1 which shows the structure of a wireless system according to the Universal Mobile Telecommunications System (UMTS is synonymous with WCDMA or wideband code division multiple access). As can be seen in FIG. 1, the UMTS architecture consists of user equipment 102 (UE which is herein synonymous with “mobile device” and “terminal equipment”), the UMTS Terrestrial Radio Access Network 104 (UTRAN), and the Core Network 126 (CN). The air interface between the UTRAN and the UE is called Uu, and the interface between the UTRAN and the Core Network is called Iu. The UTRAN consists of a set of Radio Network Subsystems 128 (RNS), each of which has geographic coverage of a number of cells 110 (C). The interface between the subsystems is called lur. Each Radio Network Subsystem 128 (RNS) includes a Radio Network Controller 112 (RNC) and at least one Node B 114, each Node B having geographic coverage of at least one cell 110 (a Node B is often referred to as a base station). As can be seen from FIG. 1, the interface between an RNC 112 and a Node B 114 is called Iub, and the Iub is hard-wired rather than being an air interface. For any Node B 114 there is only one RNC 112. A Node B 114 is responsible for radio transmission and reception to and from the UE 102 (Node B antennas can typically be seen atop tall towers or preferably at less conspicuous locations). The RNC 112 has overall control of the logical resources of each Node B 114 within the RNS 128, and the RNC 112 may also be responsible for handover decisions which entail switching a call from one cell to another or between radio channels in the same cell.
In wireless telecommunications systems, such as the system exemplified by FIG. 1, a signal to be transmitted usually has to be modulated prefatory to data transmission on the transmission channel Uu. The modulation is generally carried out by digital modulation methods which are used to transmit a desired signal on a given frequency band. If the transmitter is nonlinear there will be interference outside the frequency band allocated for signal transmission, such interference being called adjacent channel interference. Linear amplifiers cause only a little interference to adjacent frequency bands, but the power efficiency of linear amplifiers is low, and that is why some nonlinearity is often tolerated even though adjacent channel interference may result.
It is characteristic of a terminal equipment 102 in a wireless system that the terminal equipment's receiver must be able to attenuate even strong signals on an adjacent frequency band. However, adjacent channel attenuation (selectivity) by the receiver is always limited, and cannot operate with complete success when adjacent channel interference becomes large. A certain desired capability for adjacent channel attenuation value is determined for terminal equipment receivers in the system, and the receivers should attain this desired value. Therefore, each terminal equipment 102 has a particular known ability to attenuate an adjacent channel signal in order to reduce adjacent channel interference.
In some situations, adjacent channel interference increases to such an extent that the connection may be disconnected. This is called blocking of a receiver. In such a situation, it is important that an interfrequency handover can be performed rapidly and at a correct moment, in order to prevent disconnection. FIG. 2 shows the terminal equipment 102 (UE) communicating with a first base station 202 (B1) which is one of the base stations 114 in the system. Frequency F4 is used over the Uu connection in the downlink direction (B1→UE). However, the mobile device 102 is also situated close to a second base station 204 (B2), which transmits to its own mobile devices on frequency F3. In other words, the cells 110 associated with the first base station 202 and the second base station 204 overlap at the point where user equipment 102 is located. If frequencies F4 and F3 are adjacent frequency bands on the frequency range, the transmission of the second base station 204 appears to the UE 102 as adjacent channel interference, since UE's receiver selectivity is not ideal. Assume that B1 202 and B2 204 are, for example, base stations of different network operators, in which case the UE 102 cannot perform a handover to B2 204. As the interference becomes stronger, there is a risk of the connection between the UE 102 and B1 202 being disconnected.
In some prior art arrangements, a terminal equipment UE 102 measures the strengths of signals from base stations 114. The purpose of the measurements is to search for handover candidates having a strong received downlink signal, but this procedure becomes problematic in the aforementioned situation where a candidate base station 204 is located on another frequency to which the terminal equipment 102 cannot perform a handover. Furthermore, the process of directly measuring signal strengths from different base stations 114 is problematic because, even if that process occurs only at specific intervals rather than continuously, the process may intermittently detract from the terminal equipment's normal communication capacity.
In some prior arrangements, the interference caused by the terminal equipment's own transmission in the uplink direction (UE→B1) to other terminal equipments of another network operator is estimated on the basis of signal strengths measured from base station transmissions, and this is used as a basis for a handover. However, this method does not take into account the interference to the terminal equipment's own connection, which means that the handover is not carried out in the best possible manner for the terminal equipment since the call may be blocked before the handover is completed.
In another related art arrangement, a terminal equipment UE 102 communicates with a first base station B1 202 on a particular frequency band and measures that first received signal strength. Periodically, the terminal equipment 102 also measures a second received signal strength on an adjacent frequency band used by a second base station 204. If the second signal strength exceeds the first signal strength by a given threshold, then an interfrequency handover is performed so that the terminal equipment 102 continues to communicate with the first base station 202 at a different frequency. That type of arrangement is disclosed in the related art of Haemaelaeinen et al. (European patent number WO/0036867), and is also disclosed in copending U.S. application Ser. No. 09/457,918 (filed Dec. 9, 1999 and expressly incorporated by reference as background). However, that type of arrangement is problematic because a terminal equipment 102 will have lower communication capacity due to interfrequency monitoring, inasmuch as the terminal equipment 102 must regularly shift to another frequency for purposes of signal strength measurement. This problem of capacity loss can be alleviated by employing a second receiver within the same terminal equipment 102, but a second receiver would entail significant increase in cost.